The blades of a wind turbine experience loads during operation due to aerodynamic, gravitational and inertial forces. Excessive loads can damage the blades, and also other components of the wind turbine, and so it is important to ensure that the loads experienced by the blades remain within acceptable limits. In order to monitor this, modern wind turbines are usually provided with a blade load sensor system. Such a system typically includes a load sensor provided in each blade to measure the load experienced by that blade. The load sensors are usually provided in the form of a strain gauge and more particularly as an optical strain gauge, for example similar to the one described in EP1230531.
In a known type of blade load sensor system, pairs of load sensors are arranged in a cylindrical root end of the blade and function on a differential basis. An example of such a sensor setup can be found in U.S. Pat. No. 8,157,582. In such a system, typically a pair of load sensors will be located near to the internal leading and trailing edges of the blade, along the chord axis to measure the edge-wise bending moment, and a pair of sensors will be mounded to the blade on the internal leeward and windward blade surface on the flap-wise axis. This is illustrated schematically in FIG. 1, in which a cylindrical root end 2 is overlaid on a blade aerofoil section 4. The pair of flap-wise sensors 6 are positioned diametrically opposite one another and aligned on the flap-wise axis ‘F’, and the pair of edge-wise sensors 8 are also positioned diametrically opposite one another and are aligned on the edge-wise axis ‘E’.
By arranging pairs of sensors aligned on a specific axis, it is possible to determine the difference in the strain measured by each of the sensors, which ensures that inaccuracies due to axial forces acting on the blade can be factored out so that only the bending moment due to flap or edge-wise forces is detected.
However, such load sensor packages can be expensive, difficult to mount, and also problematic to maintain during the lifetime of the wind turbine, so it is desirable to provide an alternative to such load sensor packages, but without sacrificing the accuracy that is achievable with such differential sensor pairs.